After heavy rains triggered fatal landslides in 1996, Oregon rewrote its rules on where logging can happen in landslide-prone areas.
Oregon forestry rules now say you can’t log in areas with where logging could trigger a public safety risk from a certain type of landslide. But it’s not the type of landslide that devastated Oso, Wash. It’s the kind that killed people in Oregon back in 1996.
That type of landslide –- a shallow, rapid landslide or debris flow –- sends the top layer of soil washing down a slope and taking everything on the surface along with it. Removing trees from steep slopes can raise the chances of that kind of landslide, and the Oregon Department of Forestry has rules that aim to avoid that risk.
“There’s a link between harvesting and shallow, rapid landslides,” said John Seward, a geotechnical specialist who reviews logging plans for the Oregon Department of Forestry. “You can’t necessarily point to any landslide and say it was caused by logging, but our objective is to prevent logging from exacerbating those kinds of sites.”
But the landslide in Oso was a different kind of landslide, referred to as a deep-seated landslide. It happened on terrain with a long history of landslides where unstable soil extended much deeper into the side of the slope.
Peter Goldman, director of the Washington Forest Law Center, says Washington has rules restricting logging above deep-seated landslides –- in places known as “recharge zones” — but Oregon doesn’t. Logging in these drainage areas allows more water to flow into the landslide area below, and that can raise the risk of a slide.
“Oregon has no rule prohibiting logging in recharge zones above landslides,” Goldman said. “So, based on my initial review, there’s nothing in Oregon that technically would have stopped the logging of a recharge area such as what occurred up in Snohomish County.”
Seward agrees that’s true. Oregon’s rules are designed to address the more direct connection between logging and removing tree root systems that hold shallow soils in place.
He has restricted numerous logging operations after finding that they could raise the risk of a shallow, rapid landslide. But those risks don’t necessarily extend to deep-seated landslides, he said.
“In terms of harvesting with those kind of deep-seated landslides like that, they’re well beyond the rooting depth of any tree,” he said. “So if in fact root strength is a factor, that doesn’t come into play.”
Seward says the geology on the slope in Oso is different from anything in Oregon because it was created by glaciers that didn’t come down that far south.
“In Oregon we don’t have that particular landform,” he said. “So therefore we don’t have rules that address that particular situation because it doesn’t apply here.”
Oregon does have deep-seated landslides, though. Seward says even though Oregon rules don’t explicitly restrict logging above those deeper landslide areas, they do allow him and other regulators to use their judgment to flag any active landslide and restrict logging to protect public safety.